In-phase (I) and quadrature (Q) imbalance, also referred to as I/Q mismatch, is a well-known undesirable characteristic of communication equipment that implement parallel I and Q signal processing channels in transmitter and receiver circuits found in many wireless communication devices. I and Q imbalance can cause rotation, offset, skewing, and compression of data in both the transmitter and receiver of a communication system to the extent that symbol decisions at the receiver cannot be relied upon. I and Q imbalance is caused by multiple factors including inadequate phase separation in local oscillator (LO) signals, e.g., I and Q channels being other than 90° apart in phase, and circuit component differences that produce unequal signal amplitudes in the I and Q channels where equivalent signal amplitudes are required.
The state-of-the-art has advanced to allow on-chip measurement circuitry for purposes of calibrating I and Q channels in communication equipment to substantially reduce or eliminate gain and phase imbalances. In certain systems, a dedicated on-chip receiver circuit is used to downconvert a calibration signal that has been upconverted by a transmitter for purposes of calibration. However, calibration data that has traversed both a quadrature transmitter circuit and a quadrature receiver circuit contains phase and gain imbalance artifacts from both transmitter and receiver circuits. Thus, pinpointing the source of these imbalance artifacts, i.e., whether they originated in the transmitter or in the receiver cannot be readily determined from a single measurement of the affected signal.
In certain calibration techniques, one of the transmitter or the receiver is first calibrated followed by a calibration of the other of the transmitter and receiver. For example, imbalances between I and Q calibration signals may first be determined in the transmitter and the transmitter may be calibrated accordingly. Once the transmitter has been calibrated, another calibration procedure is performed for the receiver based on the knowledge that the transmitter imbalances have been corrected. In another technique, circuitry is provided to introduce multiple phase shifts between the I and Q data and the I and Q signal channels are monitored while different phase shifts are introduced. The phase shift is continually adjusted until the phase shift that minimizes the transmitter and receiver gain and phase imbalances is determined. The ordinarily skilled artisan will recognize and appreciate that these techniques require a substantial amount of time devoted to the calibration procedure, which in a manufacturing setting can greatly reduce product throughput.
Given the state of the current art, the need is apparent for a calibration technique that can determine IQ imbalance correction data from calibration data that has traversed both transmitter and receiver and that can produce such correction data in a short period of time.